Inkjet printers have print heads that operate a plurality of inkjets in each print head to eject drops of liquid ink. The liquid ink may be stored in reservoirs located in the printer, or the ink may be provided in a solid form and then melted to produce liquid ink that is supplied to the reservoirs for printing. A typical inkjet printer has one or more printheads. Each printhead typically contains an array of individual inkjets for ejecting drops of ink across an open gap to an image receiving member to form an image. The image receiving member may be a continuous web of recording media, separate sheets of media carried by a media transport system, or it may be a rotating intermediate imaging member, such as a print drum or belt. In each inkjet of a print head is an individual piezoelectric, thermal, mechanical, or acoustic actuator that generates mechanical forces that eject ink through an orifice, called a nozzle, from an ink filled conduit in response to an electrical voltage signal, sometimes called a firing signal. The amplitude, or voltage level, of the signals affects the amount of ink ejected in each drop. The firing signal is generated by a print head controller in accordance with image data. An inkjet printer forms a printed image in accordance with the image data by printing a pattern of individual ink drops at particular locations on the image receiving member. The locations where the ink drops landed are sometimes called “ink drop locations,” “ink drop positions,” or “pixels.” Thus, a printing operation can be viewed as the placement of ink drops on an image receiving member in accordance with image data.
Typically, inkjet printing does not print coverage areas as full solids so the blank pixels can be used by image processing to correct for variability in the output of the print head, such as drop variations between inkjets and weak or missing inkjets. Some types of blank pixels within characters, however, can be problematic. As used in this document, “character” means a non-graphical symbol that represents a textual element. For one, the absence of printed pixels at character edges can be visible as ragged edges. Additional issues include color registration problems between color separations and holes inside text characters. One way to address this issue is to print the text characters as solid fill.
Printing text characters with solid fill on uncoated media is effective to address these issues but printing solid fill characters on non-absorbing media presents additional problems. Non-absorbing media, especially regular coated media that are not specifically formulated for inkjet printing, typically absorb ink at a much slower rate or do not absorb ink at all. Such media include plastic sheets and flexible films. Due to the non-absorbing or slow absorbing nature of the substrates, the printed ink stays on the top surface as a highly mobile fluid, which presents various challenges to the quality of the printed images. As used in this document, the term “non-absorbing media” means media that does not absorb or slowly absorbs ink. Such non-absorbing media includes, for example, plastics, film media, and coated media. As used in this document, the term “coated media” means substrates that have been pretreated. Pretreatments include the application of a compound that prevents the absorption of liquid inks, such as the majority of compounds used to produce offset coated media. Pretreatments also include compounds that separate colorant in the ink from solvent in the ink and supports most of the colorant on the surface of the substrate. Pretreatments can absorb ink for media that is non-absorbent or, alternatively, reduce the absorption of ink by absorbent media. Pretreatments can also tailor the wetting characteristics of the ink to the media.
Some additional problems result from new inks that have been formulated for use with non-absorbing media. These new inks typically spread really well over the surface of the non-absorbing media. Before these inks are immobilized through drying, they remain on the surface of the non-absorbing media as low viscosity liquids and can puddle, spread, flow, and coalesce, which can cause print defects. The defects are especially present when the ink amount is high or when the gradient of the ink amount is high along edges. Because these inks formulated for printing non-absorbing media tend to sit on top of the non-absorbing media, far less ink is needed for area coverage than is required for printing more absorbent media. Depending on the media, the amount of ink needed for an area might be as low as 35% of the pixels within the area. This reduction in the amount of ink needed for area coverage, which is normally a benefit, can present a dilemma when printing text. If these inks are used to print solid fill characters on non-absorbing media area, then too much ink is present on the media surface and image defect issues arise. Because the inks tend to sit on top of the non-absorbing media rather than soak into the media, the puddled ink leaves the printing process vulnerable to ink coalescence effects and defects, such as grain and mottle. The puddled ink can also exacerbate bleeding, where the puddled ink finds an escape route from the printed character and forms channels outside of the character that degrade edges of the text.
Some inkjet systems have addressed the issues presented by these specially formulated inks and non-absorbing media by ejecting an additional clear ink over the media to help stop the spreading of subsequently printed inks. While effective at reducing or eliminating coalescence effects, the clear ink also reduces beneficial spreading of drops within characters and can make other defects, such as streaks, difficult to control. Improving the appearance of text characters printed with non-absorbing media inks on non-absorbing media would be a benefit for inkjet printers.